【作者】 刘平；

【导师】 张虎元；

【作者基本信息】 兰州大学 ， 地质工程， 2009， 博士

【摘要】 文物保护工程是21世纪岩土工程领域重要的发展和研究方向之一。生体建筑遗址作为我国文物的重要组成部分,是研究我国历代政治、经济、艺术、建筑、科技诸方面极为重要的形象资料。这些文物古迹不仅是中华民族五千年文明史的实物见证,也是全人类共有的珍贵宝藏。我国西北地区由于气候干旱少雨,数量众多的土建筑遗址得以保存至今,如古丝绸之路上的秦汉长城、烽燧、楼兰遗址、交河故城、高昌故城等,构成了“丝绸之路跨国联合申遗计划”中我国境内文化遗产的重要组成部分。近年来,中国国家文物局会同科技部组织并实施了一系列丝路沿线重大文物保护工程的科研立项和研究,为即将开展的大规模加固和保护工程提供重要的科学理论依据。大体量不可移动文物处于天然开放的环境中,会在自然营力的作用下经历一系列的物理、化学变化,材料的性能和功能不断退化。文物保护的实质就是延缓文物材料的老化进程。因此,文物保护加固,必须首先对文物本体风化机理以及相关影响因素的作用机制进行科学的认识,才能对不同文物、不同类型病害采取针对性的措施,真正达到对文物进行科学保护的目的。本研究得到国家科技支撑计划“土遗址保护关键技术研究”课题的资助(N2006BAK30B02)。本论文主要通过现场调查和室内试验相结合的手段,以交河故城土遗址表部干缩开裂以及结皮层的剥离脱落为研究着眼点,运用岩土工程学的理论和方法,对西北干旱地区土遗址雨蚀病害相关的风化机理进行定量化和系统化研究。首先,测得了交河故城遗址土的收缩特征曲线和土水特征曲线,从土水相互作用的物理力学机理出发,分析和阐明了遗址土的干缩开裂现象,并通过室内试验进行了模拟和验证,同时,利用连续监测和数字化图像处理等手段,确定了表面裂隙结构形态各关键影响因子间的量化关系,首次揭示了土遗址表部结皮层的形成以及剥离机制。研究结果为后续的风化病害机理研究提供了全新的理论研究方向,并为即将开展的大规模土遗址保护和加固工程提供科学理论依据。本文的主要研究内容及取得的研究成果包括：1.土作为可变形的多相孔隙介质,其失水开裂是由一系列变化过程组合而成的,包括失水干燥、体积收缩以及开裂等。研究表明,失水干燥过程中土中水的迁移方式主要分为两种,一种是由边界表面处的蒸发引起的液态水迁移,另一种是由土内孔隙中液态水的蒸发和扩散引起的水分迁移。随着水分的蒸发,土颗粒间液态水表面的水蒸气气压降低,由于土颗粒的亲水特性和其本身的几何特性,气液间压力差逐渐增大,导致颗粒间的相互吸引,土的基质经历一个持续增强的内部压缩过程,进而引起体积的收缩。2.试验结果对比以及模型拟合相关性表明,与卡尺直接测量法相比,液体石蜡法测量土的收缩曲线精度大为提高,而且试验操作简便易行。试验发现,初始饱和度较大的糊状重塑土试样,其收缩过程由初始以重力主要控制的一维竖向收缩逐渐向三维收缩发展。利用改进的测量方法,测得了三种类型土的收缩曲线,结果表明,由于土中封闭不连续的微孔中空气的存在,使得收缩曲线发生一定程度的水平偏移。土中粘粒含量以及粘土矿物类型也对收缩曲线产生一定影响,随着粘粒和膨胀性粘土矿物含量的增大,土的体积收缩率增大,而达到最终稳定状态时的孔隙比则减小。砂粒含量对土的缩限具有重要影响,试验表明,缩限随着砂粒含量的增大而降低。蒸发速率控制着收缩历时的长短,同时对孔隙比和饱和度的变化率产生重要影响,但蒸发速率并不影响土的最终体积收缩量、缩限以及缩限孔隙比。3.室内模拟试验表明,干燥过程中的边界约束是裂隙产生的必要条件。边界约束条件主要归结为三种类型的力所引发：(1)由材料边界摩擦力(或其他牵引力)或位移的产生而导致的约束条件。(2)由材料自身内部平衡应力(又称为特征应力)的集中效应所产生的约束条件。(3)由材料自身特性而产生的约束条件。对于张应力状态下裂隙的初始扩展状态,应用Griffith判别准则可以较好地对其进行模拟,在裂隙的数量、形状、分布方向等不随试样尺寸的变化而变化条件下,该准则适用于对内部裂隙随机分布条件下材料的破坏行为进行判别和分析。土的抗拉强度与吸力和饱和度之间也存在着密切的联系,同时也与Griffith判据中所涉及的临界裂隙状态相关,论文从微观的角度强调了土的结构(内部缺陷的存在)、饱和度和吸力在土的抗拉强度方面所发挥的重要作用。4.利用数字化图像处理技术,对试样表面裂隙的结构形态和分布规律进行了定量分析和研究,引入裂隙节点、区块面积、裂隙率等概念和参数,并利用统计学相关理论和方法对裂隙的有关规律进行了讨论。试验结果表明,随着试样厚度的增加,裂隙间距和裂隙条数减小,裂隙的长度和宽度则出现增大趋势。温度对干缩裂缝的形态结构同样具有重要影响,随着温度的升高,裂隙切割形成的区块数量呈减少趋势,区块的平均面积则随着温度的升高而增大,相应的裂隙长度和宽度也随之增大,但裂隙率则呈现相反的变化趋势。此外,不同的土质成分对裂隙的表面形态也具有一定的影响。5.土的质地是影响表土结皮形成的最重要因素。团聚体稳定性、表面粗糙度、矿物质和交换性离子浓度、降雨历时、降雨强度、前期含水量、湿润速度等因素同样对结皮的形成具有重要影响。土中粘粒含量为20-30%时,极易形成结皮,当粘粒含量超过40%时,团聚体变得较为稳定,结皮形成受到一定抑制。前期含水量和湿润速度对结皮形成也同样具有重要影响。粘粒含量较高的试样在较低的前期含水量情况下更容易形成结皮。湿润速度对表土结皮的影响作用随着粘粒含量的增加而增大,但在土的结构不稳定条件下,其影响作用受到很大限制。6.通过现场调查、取样和室内试验,研究了土建筑表面结皮的物理化学性质,提出了土质墙体表面结皮的形成与剥离机制。粒度分析、粉晶X射线衍射分析、易溶盐离子色谱分析表明,结皮层与下伏母墙相比,粒度偏细,易溶盐含量偏低；红外摄像测温发现,结皮层升温、降温幅度更大。研究认为,土建筑遗址结皮层的形成是墙面土体局部饱和崩解、泥浆蠕移干燥的结果,其形成初期对抵抗土遗址风化起到一定的保护作用；但随后在剧烈的热胀冷缩作用及风蚀作用影响下发生剥离和脱落,反而加速土遗址的劣化进程。111更多还原

【Abstract】 Geotechnical engineering in the 21st century is characterized by a new trend, the research on consolidation and preservation techniques for conserving the cultural relics built with geometrials. As an important component part of cultural relics, the ancient earthen architecture provides very precious information for politics, economy, art, architecture, science and technology in ancient China. The earthen architectural heritage is not only the witness for the bright history of China with a civilization five thousand years old, but also the precious heritage of the world. In northwest China, there are a large number of ancient earthen architectures left along the Silk Road, such as the Great Wall, Beacons built in Qin or Han dynasty, and the Ancient City of Loulan, Jiaohe and Gaochang, which is included in the "Transnational Joint Application Plan For Cultural Heritage in Silk Road" in China. In the last years, the State Administration of Cultural Heritage and Ministry of Science and Technology of China jointly organized and implemented a series of key scientific research projects on earthen heritage conservation for the practical consolidation and protection of the ancient earthen architectures.Exposed in the open environment, the cultural relics suffered a series of physical and chemical weathering induced by natural agency, and underwent a degradation in properties and function of construction materials. To understand the degradation process, the process of degenerate, which is the essence of the cultural relics protection, the weathering mechanism of construction materials should be firstly studied. Based on the scientific understanding for the weathering mechanism, corresponding measures can be draw up and implemented to protect different types of cultural relics.This work is part of National Key Project of Scientific and Technical Supporting Programs Funded by Ministry of Science & Technology of China (NO.2006BAK30B02). This thesis focused on the shrinkage and cracking mechanism of surface soils in the ancient city of Jiaohe, involved in the rain erosion by field investigation and laboratory test from the geotechnical engineering viewpoint. Soil shrinkage and water retention curves of earthen materials in ancient city of Jiaohe were tested, and the desiccation cracking phenomena was discussed based on the physics theories soil water interaction mechanism. Continuous monitoring system and digital image processing methods were used in laboratory test and verified so to establish a quantitative relationship between the surface cracks’pattern and their influencing factors. The generation and detachment mechanism of surface crust on ancient earthen architectures was firstly revealed. These results provide new research direction for study of weathering mechanism, and establish a theoretical basis for the large-scale protection and reinforcement projects on earthen heritage. The main contents included in this thesis are as follows:1. Soil is a deformable multiphase porous medium. The desiccation of soil is a set of processes that include drying, shrinkage and cracking. Research indicated that there were two modes of liquid transfer at the boundary of the soil sample and inside the porous space during desiccation of soil. One was purely liquid transfer induced by boundary evaporation, and the other one was liquid transfer induced by evaporation inside the body and vapor diffusion through the body. The basic process behind the shrinkage of a soil was a decrease in liquid pressure and more generally an increase in suction, caused by the evaporation at the level of menisci, generating an additional attractive force between the components of the matrix. Therefore, an increasing internal compression and the matrix shrink of the sample occurred.2. An improved method for volume measurement, determining soil shrinkage characteristic curves (SSCC), was proposed. Through comparison of test and model fitting results, the paraffin oil method, tending to narrow the measurement error, is a better alternative to describe the SSCC of structure-less clay paste than the caliper method. The observation of geometry factor indicated that, at the beginning of desiccation, the clay paste was controlled only by a vertical downward movement of soil particles due to gravity. With further drying, the soil became stable and the horizontal shrinkage component increases, resulting in a three dimensional shrinkage. Using the improved volume measurement, the soil shrinkage characteristic curves of different soils were measured and determined. The results showed that, due to the presence of some non-connected pores which were not filled by free water, a shift of the shrinkage curve was found. Results of the investigation also indicated that the different soil samples showed different shrinkage properties according to clay type and clay content. The shrinking capacity of the specimen volume increased with the content of clay, but there was an inverse proportionality between the clay content and the void ratio at shrinkage limit. Test results also indicated that the shrinkage limits decreased with the content of sand in soils, and that the rates of changes in void ratio and degree of saturation of the soils were highly affected by the evaporation rates. However, the shrinkage curves of the soils, was not affected by the drying rates. An identical shrinkage curve was obtained for the same soil under different drying rates. Similarly, the relationship between void ratio and matrix suction for the same soil under different drying rates was also identical.3. Experimental model of cracking showed that desiccation macro-cracks in soil were the consequence of constrained drying shrinkage and tensile stress generation that exceeds soil tensile strength. Typically, three kinds of constraints responsible for the desiccation can be identified:(1) a frictional or any other traction or displacement boundary conditions; (2) any eigen-stress concentrations within the soil sample; (3) intrinsic factors, such as soil texture and soil structure. Research indicated that the Griffith’s criterion (defined in terms of macroscopic tensile strength) was useful for modelling the strength of a volume of an elementary representative of soil submitted to tensile states. Griffith’s criterion provided a fundamental basis for such a sample that has randomly distributed flaws in case that considering the hypothesis that the number, shape, and orientation of flaws should not change with the sample size. Tensile strength, depended on the degree of saturation and on suction, was also related to the most critical flaw in the Griffith sense. Besides, from a microscopic point of view, the study emphasized the important role of soil structure (such as the presence of defects), degree of saturation and suction.4. The computer image processing technique was applied to quantitatively analyze and describe the structure and geometric characteristics of crack. Some factors influencing the shrinkage, such as temperature, sample thickness and composition of soil, were taken into account. The test results showed that with the increase of sample thickness, the space between soil cracks were reduced, while the length and width of cracks were increased. The crack length, width, aggregate area and their most probable value were related to the probability density functions, which tend to increase with temperature increase. With a thicker soil layer, the average crack length, width, aggregate area and crack intensity factor increased. It was also observed that the effect of soil types on the crack pattern should not be neglected. In addition, it was observed that cracking occurred at three stages and the water loss rate increased after the shrinkage cracks had appeared on the surface of soil samples.5. The formation of soil surface crust mainly depends on soil permanent properties, such as soil texture, mineralogy, organic and inorganic polymers, composition of exchangeable cations, aggregate stability, antecedent water content and wetting rate. The aggregate stability and clay content have substantial effects on formation of surface crust. Research shows that soils with 20-30% clay were the most susceptible to crust formation. With clay content above 40%, soil structure became more stable, and crust formation was restrained. In addition, smectite soils, low levels of exchangeable sodium percentage and electrolyte concentration were all beneficial to the formation of soil surface crust. Wetting rate, antecedent moisture content, also significantly affect soil susceptibility to crusting, but this was often neglected by many researchers. Fast wetting of dry soil caused aggregate slaking and crusting whereas high antecedent moisture content decreased aggregate disintegration.6. Field survey, systematic sampling and laboratory tests were conducted to study the weathering mechanism of earthen architecture taking the Jiaohe Relics site, Xinjiang, China as an example. Particle size analysis, X-ray diffraction and chemical analysis of soluble salts illustrate that surface crusts are characterized by finer particles and lower soluble salts contents compared with the host soil. Temperature monitoring by IR thermography camera and thermal sensors shows that thin crusted layers response greatly and quickly to the environmental temperature change. It is concluded that the local saturation and slurry film generation due to rainfall are mainly responsible for the formation of surface crusts; and the crust will be subsequently detached by wind erosion and expansion-contraction induced by the change of temperature. Surface crusted layer prevents the wall from weathering to some degree at its earlier formation stage but tend to accelerate the deterioration of the earthen architectures in the long term considering that Jiaohe site suffer from strong sandstorm and strict weather condition.更多还原